Eric Hebborn had been working on the manuscript, titled The Language of Line, when he was found dead in a Roman street in 1996.
Photograph: BRENDAN BEIRNE/REX Shutterstock

Eric Hebborn, the great forger who 20 years ago met a brutal end, duped art experts worldwide with his brilliant, fake old masters. Now, from beyond the grave, he has taken yet another swipe at those same experts.

In an unpublished drawing manual that has come to light, Hebborn ridicules the “threadbare terminology of traditional connoisseurship” and dismisses experts as largely unequipped to discuss drawings beyond superficial observations.

Hebborn had been working on the manuscript, titled The Language of Line, when he was found dead in a Roman street in 1996 after suffering a severe blow to the head. Whether he was murdered remains unclear. But he certainly made enough enemies, having fooled some of the world’s foremost galleries and auction houses with forgeries in the style of Rubens and Van Dyck, among others.

He writes: “On the whole, critics, connoisseurs and art historians, when obliged to go beyond statements of fact – such as authorship, subject matter, measurement and medium – and speak of the quality of the drawing, tend to do so in the vaguest of terms.”

Hebborn mocks their use of simplistic words such as beautiful, powerful and magnificent “when praising”, or ugly, feeble and poor “when damning”: “Terms of this kind are open to far too wide an interpretation.”

He also argues that draughtsmanship can only be understood by those who “can, to some extent at least, draw”: “I know this is not a very palatable idea to those authorities who seem to know everything there is to know about drawing – except how to do it …

“To imagine that one can learn about drawings simply by looking at them is presumptuous, not unlike imagining that one can learn how aeroplanes work simply by flying in them.”

Despite winning prizes as a student at the Royal Academy Schools, Hebborn later failed to find recognition as an artist in his own right. As an art restorer, he acquired a knowledge of the old masters’ materials and methods “until I would one day be able to ‘restore’ a whole painting, from nothing at all”, he later recalled.

A typed copy of his drawing manual was shown to the Guardian by Brian Balfour-Oatts, an art dealer who acted as Hebborn’s agent in his final two years, buying the rights to his 1991 autobiography, Drawn to Trouble.

The drawing manual was given to him by one of Hebborn’s friends after hearing that he had been outbid for a handwritten copy at auction last October by Webbs of Wilton in Wiltshire. It went to an anonymous bidder for £3,600, vastly exceeding its £50 estimate.

Hebborn’s damning views of art experts are particularly topical.

In December, ArtWatch UK, the art world watchdog, will stage a conference on connoisseurship at the London School of Economics. The ArtWatch director, Michael Daley, who also trained as an artist at the Royal Academy Schools, said: “Hebborn has a point about the need for first-hand knowledge of the practice of draughtsmanship. But there is a problem today in that drawing has all but disappeared from the training of fine artists.”

He speculated that the hand-written drawing manual might have been bought by someone who assumed it held vital clues to forgeries that were hanging undetected in public and private collections.

The manual appears to be a treatise on drawing. While Hebborn’s previous book, The Art Forger’s Handbook, revealed tricks of his trade, this one focuses on an art form in need of what he calls an urgent rescue operation.

Tantalisingly, the text repeatedly refers to absent drawings. Balfour-Oatts believes that Hebborn had not completed them before his death.

He described Hebborn as “an old master at the wrong time”, an artist who was extremely knowledgeable and who had “obviously gone through periods where he’d been extremely bitter”. He suspects that, long after being unmasked, Hebborn was still producing forgeries.

Eight “Hebborns” have been registered with the Art Loss Register (ALR), whose database of fakes and forgeries has doubled to 12,000 in the last two years.

They include his “Van Dyck” – Christ Crowned with Thorns – which entered the British Museum in 1970, following a purchase from the leading old master specialist gallery, Colnaghi. The museum’s website notes: “It was first suspected of being a fake by Eric Hebborn in 1978, and after he had stated that it was his work in a letter to Mark Jones in 1990, it was re-attributed and transferred to the Fake box in January 1991.”

Will Korner of ALR said: “We used to have these gentlemen art forgers, who were very proud of their own work. Recent cases suggest that the age of individual master-forgers is to be replaced by mass-production of fakes for sale online from China and elsewhere.”